The US government has been accused of misleading a British minister over the brutal treatment endured by the last British resident being held inside Guantánamo Bay.

Testimony from detainees has described increasingly violent “forcible cell extraction” (FCE) tactics, in which an inmate is forced out of his cell by armed guards, usually before being taken to the force-feeding chair.

Earlier this month a federal judge, Gladys Kessler, heard how methods used by the US military to feed inmates against their will present long-term health risks and that lubricating their feeding tubes with olive oil can cause chronic inflammatory pneumonia.

However, attempts by the British government to establish if Shaker Aamer, whose family are in south London, has been mistreated appear to have been dismissed. The foreign secretary, Philip Hammond, revealed in a letter dated 7 October: “We made inquiries with US government officials, who assured us that the report of an incident, relayed to you by another detainee, is not accurate.”

The response relates to claims from a fellow detainee, Yemeni Emad Hassan, who reported that Aamer had been “beaten” by FCE teams. Aamer, 46, has described being beaten by the FCE team up to eight times a day.

Insisting that Aamer’s release remains a high priority for the government, Hammond said UK officials have no access to the British resident and are solely reliant on US sources for information. Clive Stafford Smith, director of legal charity Reprieve, who recently visited Aamer in Guantánamo, said: “It is unfathomable to me that the British government would blithely accept the US assurances that all is well in Guantánamo when they are not allowed to visit Shaker, when judge Kessler has recently seen and heard the evidence that prisoners are being terribly abused, and when the International Committee of the Red Cross itself has characterised some of the ‘techniques’ being used in Guantánamo as torture.”

Aamer, who has been cleared for release by both the Bush and Obama administrations, has been held for long periods of solitary confinement since 2005 and is in extremely poor health.

An independent medical examination, published this year, diagnosed severe post-traumatic stress and recommended urgent psychiatric treatment for him and “reintegration into his family”.